This application claims priority to United Kingdom (GB) patent publication number GB 0016271.9 filed on Jul. 4, 2000.
This invention concerns improvements in or relating to vehicle braking systems and in particular, though not exclusively, to braking systems for agricultural vehicles such as tractors and the like.
Agricultural vehicles are commonly provided with braking systems employing xe2x80x9cannular piston brakesxe2x80x9d in which fluid contained in an axle is pressurized when a brake pedal is depressed to actuate a piston to apply the brake to a rear wheel.
Normally, separate brake pedals are provided for applying the brakes to the rear wheels on opposite sides of the vehicle. In this way, the rear wheels can be braked independently to facilitate steering by depressing the brake pedal associated with the brake for that rear wheel or simultaneously for full vehicle retardation by depressing both brake pedals.
Agricultural vehicles, especially tractors, typically include a source of pressurized hydraulic fluid for operating ancillary equipment and it is known to employ this in a booster to boost an input force producing an amplified output force to actuate the brakes. In this way, braking performance, especially for full vehicle retardation, is enhanced.
The known boosters typically have a boost piston working in a bore in a body to apply an output force in response to a pressure applied to a power chamber under the control of a control valve in turn responsive to an input force.
The input force is that generated by actuation of the brake pedal which operates the control valve to connect the power chamber to the source of pressurized hydraulic fluid which acts to displace the boost piston and produce the amplified output force for actuating the brakes.
Conventionally, the boosters are installed at a remote location from the brakes and are typically mounted to the cab of the vehicle requiring long hydraulic pipe runs to and from the vehicle cab to connect the boosters to the brakes and the available supply of pressurized fluid.
Such pipe runs add to installation costs and lead to significant noise levels being generated by flow of hydraulic fluid along the pipe runs to and from the vehicle cab when the braking system is operated.
Furthermore, the pipe runs are often exposed and there is a risk of damage to the pipe runs with loss of hydraulic fluid and increased risk of injury from failure of the braking system.
Moreover, the remote location of the booster at the cab can lead to responsiveness and hysteresis problems with the relatively high volumetric consumption of some annular piston brakes.
The present invention has been made from a consideration of the foregoing problems and disadvantages of known braking systems.
Thus, it is an object of this invention to provide a braking system employing a booster to boost an input force and generate an amplified output force for actuating a brake in which the pipe runs connecting the booster to the brake are reduced and more preferably eliminated.
It is a further object of this invention to provide a braking system employing a booster in which the noise levels generated by operation of the braking system are reduced.
It is yet another object of this invention to provide a braking system employing a booster in which installation of the booster is simple and access to the booster for repair/maintenance is facilitated.
These and other objections of the invention are generally provided by an arrangement in which a booster for a brake is relocated from the cab to the brake for a wheel to be braked.
Thus, according to a first aspect of this invention there is provided a vehicle braking system including a booster to boost an input force and generate an amplified output force to actuate a brake for a wheel of the vehicle in which the booster and brake are combined in an integrated unit.
By combining the booster with the brake, the output force from the booster acts directly to actuate the brake. In this way, long pipe runs to connect the booster to the brake are eliminated and operating noise levels are reduced. Also, the responsiveness and hysteresis problems of existing cab mounted boosters are substantially eliminated.
Moreover, a simple actuation system may be employed for the booster such as a brake pedal operating a master cylinder to provide the input force to the booster.
Furthermore, a self-bleeding facility for the brake hydraulics may be provided via the booster. This has particular benefit for application of the invention to hydrostatic drives and simplifies connection to the available hydraulic supply.
Additionally, a pre-assembled axle assembly with integral hydraulic booster brakes may be provided. This has advantages for the manufacture, installation and subsequent maintenance of the axle assembly.
Preferably, the brake includes a piston slidable in a bore to apply the braking force under the control of fluid pressure in a chamber responsive to the amplified output force of the booster.
The brake may be an annular piston brake. In this case the chamber is provided within an axle casing for the wheel to be braked and the booster is mounted on the casing so that the amplified output force acts directly on the fluid in the chamber.
The booster may have a boost piston working in a bore to generate the amplified output force in response to a pressure applied to a power chamber under the control of a valve, in turn responsive to the input force. In this case, the valve controls communication between the power chamber and either one of a reservoir tank and a supply of pressurized fluid in response to the input force. For example, the valve may be arranged to control ports connecting the power chamber to the reservoir tank and the pressurized fluid supply.
Preferably, a reservoir port is open and a supply port closed in a rest or passive condition of the booster to isolate the power chamber from the fluid supply with the valve being operable to close the reservoir port and open the supply port in response to the input force to connect the power chamber to fluid supply
In this way, the pressure of the fluid admitted to the power chamber advances the boost piston to boost the input force and generate the amplified output force for actuating the brake.
In a preferred arrangement, the valve comprises a spool working in a bore in the boost piston to control opening and closing of the ports connecting the power chamber to the reservoir and fluid supply in response to the input force acting on the spool.
Preferably, the spool is biased to a retracted position in the rest or passive condition of the booster and the input force acts to displace the spool against the biasing.
Advantageously, the biasing is provided by a return spring within the bore of the boost piston in which the spool works, and a stop is provided to limit movement of the spool and define an end position preventing further separation of the boost piston and spool under the biasing of the spring.
Preferably, the supply port is closed and the reservoir port is open when the spool engages the stop in the end position. For example, when both the boost piston and spool are in their retracted positions. In this way, the fluid pressure supply is isolated from the power chamber when the spool engages the stop.
Advantageously, the booster is provided with a bleed hole connecting the chamber to the reservoir tank in the rest or passive condition of the booster, and the bleed hole is isolated from the chamber when the boost piston is advanced.
Preferably, the boost piston is biased to a retracted position in the rest or passive condition of the booster and the bleed hole connects the chamber to the reservoir tank when the boost piston is in the retraced position with initial movement of the boost piston from the retracted position isolating the bleed hole from the chamber. In this way, the chamber is self-bleeding when the brake is inoperative with the boost piston retracted.
The preferred position of the booster for the chamber to be self-bleeding is at the top of the chamber with the booster extending vertically. The booster may be arranged to extend other than vertically depending on the available space and other requirements of the installation. For example, the booster may be arranged to extent at an angle up to 90xc2x0 from the vertical, i.e., almost horizontally.
According to a second aspect of the invention, there is provided a booster for a vehicle braking system having a boost piston working in a bore and adapted to generate an output force in response to a pressure applied to a power chamber under the control of a valve which in turn is responsive to an input force wherein the booster is adapted for mounting on an axle having a piston working in a bore to apply a braking force in response to the output force generated by the booster.
According to a third aspect of the invention, there is provided a vehicle having a braking system according to the first and second aspects of the invention.
Preferably, the braking system is arranged to apply the braking force to a rear wheel on one side of the vehicle under the control of an actuation system such as a brake pedal operating a master cylinder to actuate the booster.
Advantageously, the vehicle has further rear wheel on the other side with a further braking system and actuation system for that wheel. In this way, full vehicle retardation is provided by operating both brake pedals simultaneously to apply the brakes on both sides while steering may be facilitated by operating either one of the brake pedals separately to apply the associated brake on that side only.
Other advantages of the axle mounted booster according to this invention include the capability to provide a simple master cylinder actuator from the cab to the booster in a pre-assembled axle assembly with integral hydraulic booster brake which is simple to install.
These and other benefits arising from the re-positioning of the booster from the cab to the brake will be further explained and described later herein with reference to an embodiment of the invented braking system.